My shower has been leaking BEHIND the wall! This happens only when the shower valve is on. I took the cover off the shutoff valve and pulled the escutcheon back from the wall. I took the shower head off and installed a cap. I turned the shower on and with a flashlight I could see it dripping behind the wall. I removed the shower arm and installed a plug and when I turned on the shower, there were no drips! I then took the plug out, cleaned up the threads, put new teflon tape on the shower arm and re-installed the arm. Turned on the shower and it started dripping again! The shower arm threads don't look damaged, but for some reason it just won't seal up. Am I doing something wrong? Is it possible to over tighten the shower arm? Should I get a new one? Should I use thread sealer instead of teflon tap? HELP......